1. Field of the Invention
This invention relates generally to apparatus and methods for monitoring parameters associated with the circulatory system of a living subject, and specifically in one embodiment to the non-invasive monitoring of e.g., arterial blood pressure.
2. Description of Related Technology
The accurate, continuous, non-invasive measurement of blood pressure has long been sought by medical science. The availability of such measurement techniques would allow the caregiver to continuously monitor a subject's blood pressure accurately and in repeatable fashion without the use of invasive arterial catheters (commonly known as “A-lines”) in any number of settings including, for example, surgical operating rooms where continuous, accurate indications of true blood pressure are often essential.
Several well known techniques have heretofore been used to non-invasively monitor a subject's arterial blood pressure waveform, namely, auscultation, oscillometry, and tonometry. Both the auscultation and oscillometry techniques use a standard inflatable arm cuff that occludes the subject's brachial artery. The auscultatory technique determines the subject's systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject's mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject's actual blood pressure waveform. Thus, true continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.
Occlusive cuff instruments of the kind described briefly above have generally been somewhat effective in sensing long-term trends in a subject's blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.
The technique of arterial tonometry is also well known in the medical arts. According to the theory of arterial tonometry, the pressure in a superficial artery with sufficient bony support, such as the radial artery, may be accurately recorded during an applanation sweep when the transmural pressure equals zero. The term “applanation” refers generally to the process of varying the pressure applied to the artery. An applanation sweep refers to a time period during which pressure over the artery is varied from overcompression to undercompression or vice versa. At the onset of a decreasing applanation sweep, the artery is overcompressed into a “dog bone” shape, so that pressure pulses are not recorded. At the end of the sweep, the artery is undercompressed, so that minimum amplitude pressure pulses are recorded. Within the sweep, it is assumed that an applanation occurs during which the arterial wall tension is parallel to the tonometer surface. Here, the arterial pressure is perpendicular to the surface and is the only stress detected by the tonometer sensor. At this pressure, it is assumed that the maximum peak-to-peak amplitude (the “maximum pulsatile”) pressure obtained corresponds to zero transmural pressure.
One prior art device for implementing the tonometry technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue, to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to at least some of the transducers. An array of different transducers is used to ensure that at least one transducer is always over the artery, regardless of array position on the subject. This type of tonometer, however, is subject to several drawbacks. First, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject's tissue overlying the artery being sensed. This has historically led to inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and nerve damage and can restrict blood flow to distal tissue.
Other prior art techniques have sought to more accurately place a single tonometric sensor laterally above the artery, thereby more completely coupling the sensor to the pressure variations within the artery. However, such systems may place the sensor at a location where it is geometrically “centered” but not optimally positioned for signal coupling, and further typically require comparatively frequent re-calibration or repositioning due to movement of the subject during measurement. Additionally, the methodology for proper initial and follow-on placement is awkward, essentially relying on the caregiver to manually locate the optimal location for sensor placement on the subject each time, and then mark that location (such as by keeping their finger on the spot, or alternatively marking it with a pen or other marking instrument), after which the sensor is placed over the mark.
Tonometry systems are also commonly quite sensitive to the orientation of the pressure transducer on the subject being monitored. Specifically, such systems show a degradation in accuracy when the angular relationship between the transducer and the artery is varied from an “optimal” incidence angle. This is an important consideration, since no two measurements are likely to have the device placed or maintained at precisely the same angle with respect to the artery. Many of the foregoing approaches similarly suffer from not being able to maintain a constant angular relationship with the artery regardless of lateral position, due in many cases to positioning mechanisms which are not adapted to account for the anatomic features of the subject, such as curvature of the wrist surface.
Another deficiency of prior art non-invasive hemodynamic measurement technology relates to the lack of disposability of components associated with the device. Specifically, it is desirable to make portions of the device which may (i) be contaminated in any fashion through direct or indirect contact with the subject(s) being monitored); (ii) be specifically calibrated or adapted for use on that subject; (iii) lose calibration through normal use, thereby necessitating a more involved recalibration process (as opposed to simply replacing the component with an unused, calibrated counterpart), or (iv) disposable after one or a limited number of uses. This feature is often frustrated in prior art systems based on a lack of easy replacement of certain components (i.e., the components were not made replaceable during the design process), or a prohibitively high cost associated with replacing components that are replaceable. Ideally, certain components associated with a non-invasive hemodynamic assessment device would be readily disposable and replaced at a very low cost to the operator. Yet another disability of the prior art concerns the ability to conduct multiple hemodynamic measurements on a subject at different times and/or different locations. For example, where blood pressure measurements are required in first and second locations (e.g., the operating room and recovery room of a hospital), prior art methodologies necessitate either (i) the use of an invasive catheter (A-line), (ii) transport of the entire blood pressure monitoring system between the locations, or (iii) disconnection of the subject at the first monitoring location, transport, and then subsequent connection to a second blood pressure monitoring system at the second location.
The disabilities associated with invasive catheters are well understood. These include the need to perforate the subject's skin (with attendant risk of infection), and discomfort to the subject.
Transport of the entire blood pressure monitoring system is largely untenable, due to the bulk of the system and the desire to maintain monitoring equipment indigenous to specific locations.
Disconnection and subsequent reconnection of the subject is also undesirable, since it requires placing a sensor or apparatus on the patient's anatomy a second time, thereby necessitating recalibration, and reducing the level of confidence that the measurements taken at the two different locations are in fact directly comparable to one another. Specifically, since the sensor and supporting apparatus is physically withdrawn at the first location, and then a new sensor subsequently placed again on the subject's tissue at the second location, the likelihood of having different coupling between the sensor and the underlying blood vessel at the two locations is significant. Hence, identical intra-vascular pressure values may be reflected as two different values at the different locations due to changes in coupling, calibration, sensor parameters, and related factors, thereby reducing the repeatability and confidence level associated the two readings.
Another disability of the prior art relates to the lack of any readily implemented and reliable means or mechanism for correction of blood pressure readings for differences in hydrostatic pressure resulting from differences in elevation between the pressure sensor and the organ of interest. For example, where a surgeon or health care provider wishes to know the actual pressure in the brain or head of the subject, the pressure reading obtained from another location of the body (e.g., the radial artery) must be corrected for the fact that the subject's blood volume exerts additional pressure at the radial artery, presumed to be lower in elevation than the subject's head. The additional pressure is the result of the hydrostatic pressure associated with the equivalent of a “column” of blood existing between the radial artery and the uppermost portions of the subject's anatomy.
Additionally, differences in pressure resulting from hydrodynamic effects associated with the cardiovascular system. While quite complex and sophisticated, the circulatory system of a living being is in effect a piping system which, inter alia, generates flow resistance and therefore head loss (pressure drop) as a function of the blood flow there through. Hence, significant difference between the pressures measured at the output of the heart and the radial artery may exist due to purely hydrodynamic effects.
Prior art techniques for correcting for hydrostatic pressure difference generally comprise measuring the difference in elevation between the measurement location and the organ of interest, and then performing a manual or hand calculation of the hydrostatic pressure correction resulting from this difference, based on an assumed gravitational field vector magnitude g (commonly rounded to 9.8 m/s2). Such techniques are cumbersome at best, and prone to significant errors at worst.
Yet another disability with many prior art systems relates to their bulk and size, especially those portions of the apparatus affixed to the subject; e.g., at the radial artery. Simply stated, the smaller and lighter the device, the greater the degree of flexibility with respect to orientation and placement of the subject and their limbs. For example, certain types of surgical procedures require one or more arms of the subject to be placed tightly at their side(s), which can be difficult where the blood pressure measurement device disposed on the radial artery is bulky and heavy. Similarly, when it is desired to “dangle” the subject's arm, excessive weight of such prior art devices can cause undue stress on and perhaps even injury to the subject being monitored. The weight of the apparatus also tends to change the physical dynamics of the non-invasive measurement process, thereby reducing accuracy and repeatability.
Furthermore, such bulk can cause the caregiver to place the subject's anatomy in a specific orientation (which may or may not be optimal) to accommodate the apparatus. This, coupled with typically several wires or cords running from the apparatus to a parent monitor or other device, makes the use of the prior art apparatus difficult in certain circumstances, and can significantly add to the clutter of an already crowded surgical area, thereby restricting movement of both the subject and any surgical or caregiver personnel.
Based on the foregoing, there is needed an improved apparatus and methodology for accurately, continuously, and non-invasively measuring blood pressure within a living subject. Such improved apparatus and methodology would ideally allow for prompt and accurate initial placement of the tonometric sensor(s), while also providing robustness and repeatability of placement under varying patient physiology and environmental conditions. Such apparatus would also incorporate low cost and disposable components, which could be readily replaced in the event of contamination or loss of calibration/performance (or purely on a preventive or periodic basis). A very small, compact and lightweight form factor is also highly desired, and a lack of direct (wired) electrical connections is ideal in certain applications.
Such apparatus and methods would furthermore be easily utilized and maintained by both trained medical personnel and untrained individuals, thereby allowing certain subjects to accurately and reliably conduct self-monitoring and maintenance of the system.